New orally active enkephalinase inhibitors: their synthesis, biological activity, and analgesic properties Original Research Article

A series of (4S)-4-[(2S)-benzyl-3-mercaptopropionylamino]-4-(N-phenylcarbamoyl)-butyric acids has been identified as potent systemically active enkephalinase inhibitors. Structure–activity relationships (SAR) are discussed. Further chemical modification of the inhibitors was carried out in order to identify the inhibitors which are orally active in an animal model. Compounds of particular interest are the prodrug-like analogues, including 5b (ONO-9902). Their analgesic effects after oral administration were evaluated.